SICP习题练习

练习1.6

new-if的三个参数会先被执行，这样就会无限循环下去

练习1.7

(define (sqrt-iter last-guess guess x)
  (if (good-enough? last-guess guess)
    guess
    (sqrt-iter guess (improve guess x) x))

另一种解法：

(define (good-enough? guess x) 
  (< (/ (abs (- (square guess) x)) guess) (* guess 0.0001)))